1. Field of the Invention
The present invention relates to a spread spectrum communication system and in particular, in a quadrature phase shift keying (QPSK) system, an object thereof is to provide a spread spectrum transmitter, spread spectrum receiver and spread spectrum communication method wherein mutual interference of the Q (quadrature) component and I (in-phase) component due to carrier frequency error is reduced. A further object is to provide a spread spectrum receiver in which the bit width of the demodulator is reduced and an automatic gain control circuit for the spread spectrum receiver.
2. Description of the Related Art
Of methods of generating a spread signal in spread spectrum, the spreading method called "direct sequence (DS)" involves spreading the spectrum of the transmission signal using a spreading code called a pseudo-random code. In general, spread spectrum modulation using the QPSK system may be divided into two stages, namely, primary modulation and secondary modulation. The primary modulation consists in quadrature phase modulation in which the raw digital data prior to modulation is divided into baseband signals that take values of .+-.1 every 2 bits and the respective divided data are modulated by a carrier wave offset in phase by 90.degree.. The respective baseband signals are called the I signal and Q signal. In secondary modulation, the primary modulation signals (I signal and Q signal) obtained by quadrature phase modulation are respectively multiplied by PN sequence codes supplied from a PN sequence generator to produce a spread spectrum. In this context, a PN sequence means a random square wave that changes with a rate that is higher than the bit rate of the raw data. The duration of a single bit in the pseudo-random signal of this PN sequence is called one chip, and the length of this period is referred to as the chip duration (hereinbelow denoted by Tc). The secondary modulated transmission signals are transmitted after superimposed.
In general, a spread spectrum transmission signal that is transmitted by the QPSK system is demodulated by the following procedure. Specifically, the received signals are amplified by a front end amplifier and subjected to quadrature detection with carrier waves mutually different in phase by 90.degree. by a quadrature detection circuit, being thereby converted into an Ir signal and Qr signal. Despreading is effected by multiplying these two baseband signals with spreading code supplied from a PN sequence generator identical with that used in transmission. In performing despreading, the phase difference (called the phase error) between the quadrature axis of the baseband signal contained in the transmission signal and the quadrature axis of the baseband signal obtained by quadrature detection must be minimised. Also, in clock recovery, synchronisation is established in the initial period of communication. Once synchronisation has been established, despreading of the PN sequence can be continued with the phase on synchronisation maintained using a delayed lock loop circuit (DLL).
Techniques of this type are disclosed in the following prior art references.
Japanese Patent Laid-Open (Kokai) No. H5-344093 discloses a technique in which a clock is recovered by detecting the peak of the sum of squares of the in-phase component and quadrature component of the output signal of a sliding correlator.
Japanese Patent Laid-Open (Kokai) No. H6-232838 discloses a spread spectrum receiver for obtaining a modulation system such that the amplitude of the spread spectrum transmission waveform cannot become 0.
Japanese Patent Laid-Open (Kokai) No. H6-244820 discloses a technique in which synchronisation capture and frequency error correction are performed by employing a plurality of despreading correlators.
Japanese Patent Laid-Open (Kokai) No. H7-107007 discloses a spreading code generating system in spread spectrum communication.
Japanese Patent Laid-Open (Kokai) No. H.7-131379 discloses a demodulating device for a spread spectrum communications receiver that demodulates a BPSK signal.
Japanese Patent Laid-Open (Kokai) No. H.7-140244 discloses a technique for synchronisation capture by sliding the phase of the spreading code until the correlation of the received baseband signal and spreading code is a prescribed value.
Japanese Patent Laid-Open (Kokai) No. H.7-283762 discloses a technique in which an SAW matched filter is employed as demodulator.
Japanese Patent Laid-Open (Kokai) No. H.7-288511 discloses a technique in which the frequency of a voltage-controlled oscillator is controlled by applying attenuation to the output of a correlator.
Japanese Patent Laid-Open (Kokai) No. H.8-149048 discloses a technique in which an SAW matched filter is employed in a modulation device and demodulator using the offset QPSK modulation system.
Also, other references include the following.
Japanese Patent Laid-Open (Kokai) No. H.3-101534 discloses a direct-sequence spread spectrum communication system and receiving device wherein quasi-synchronous detection is performed in order to make possible implementation of the circuit in LSI form by lowering the frequency band width of signal processing.
Japanese Patent Laid-Open (Kokai) No. H.7-183831 discloses a digital communications method and digital communications device in which equivalent multi-bit quantization and equivalent demodulation are performed by sampling by single-bit quantization of a direct-sequence modulated received signal. This device does not require an analogue-digital converter (A/D) or automatic gain control circuit (AGC).
Japanese Patent Laid-Open (Kokai) No. H.8-8782 discloses a direct-sequence spread spectrum communication system in which the input amplitude level of an A/D converter is optimised in response to C/N of the received signal by setting the value of the quantization level with respect to the input signal in A/D to a value adapted to the C/N; it is equipped with an AGC circuit.
Japanese Patent Laid-Open (Kokai) No. H.8-32487 discloses a spread spectrum communication system in which amplitude information identical with the QAM modulation system is conferred by applying a spread spectrum self-correlation characteristic, using a phase modulation system of fixed carrier amplitude.
Incidentally, Japanese Patent Laid-Open (Kokai) No. H.5-344093 discloses a technique of clock recovery by detecting the peak of the sum of squares of the output signal of a sliding correlator of the in-phase component and quadrature component, but, with this technique, if carrier frequency error or carrier phase error is present, clock recovery cannot be achieved.
Also, Japanese Patent Laid-Open (Kokai) No. H.6-244820 discloses a technique in which synchronisation capture and frequency error are performed by employing a plurality of despreading correlators; however, its layout is complicated since the number of correlators is doubled due to the use of a plurality of types of spreading code.
However, with a direct sequence spread spectrum system, although a correlator such as DMF (Digital Matched Filter) or SC (Sliding Correlator) is employed when despreading with the baseband at the receiving end, since these correlators perform addition of (spreading code length).times.(oversampling number), there was the problem that the bit width of the output was increased. Accompanying this, there was the problem that the scale of the hardware became large, increasing power consumption.